FIG. 6 is a circuit diagram illustrating a switching power apparatus 20 in the related art. The switching power apparatus 20 includes a switching output stage 21 configured to generate an output voltage VOUT by stepping down an input voltage VIN by driving a coil 213 in response to ON/OFF operations of an output transistor 211, a control circuit 22 configured to drive the switching output stage 21 by generating a gate signal G1 of the output transistor 211, and an overcurrent protection circuit 23 configured to perform an overcurrent protection operation by monitoring an coil current IL flowing through the coil 213 of the switching output stage 21.
Furthermore, either a pulse-by-pulse operation or an off-latch operation is generally employed as an overcurrent protection operation. In the pulse-by-pulse operation, a forced stop and a self-reset of a switching operation are repeated in each period. Therefore, even if an overcurrent is detected and an operation for switching the output transistor 211 is forcefully stopped in a certain period, an operation for switching the output transistor 211 is self-reset (self-return or resumed) in a next period. Incidentally, in an off-latch operation, if an overcurrent is detected, an operation for switching the output transistor 211 is forcefully stopped for at least a plurality of periods (in some cases, until power is applied again or an external reset is performed).
FIGS. 7A and 7B are diagrams illustrating merits and demerits of a pulse-by-pulse operation. As described above, in a pulse-by-pulse operation, even if an overcurrent is detected and the output transistor 211 is forcefully turned OFF in a certain period, change of an output voltage VOUT associated with an overcurrent protection operation may be suppressed to be relatively small since the output transistor 211 is essentially turned ON in a next period. Therefore, even if an overcurrent protection is temporarily activated due to a change of an AC load, the output voltage VOUT may be maintained (refer to FIG. 7A).
However, if, for example, an output terminal of the output voltage VOUT is ground-faulted via a low impedance path (short-circuit to a ground terminal or a low potential terminal equivalent thereto), a decrease AIL of a coil current IL during a forced OFF period of the output transistor 211 (determined in correspondence to VOUT/L) is small and the forced OFF period of the output transistor 211 is short (a period of a switching operation at the maximum), overcurrent protection cannot be sufficiently activated and the coil current IL continues to increase (refer to FIG. 7B).
FIGS. 8A and 8B are diagrams illustrating merits and demerits of an off-latch operation. As described above, in an off-latch operation, the output transistor 211 is forcefully turned OFF for a long period after detection of an overcurrent. Therefore, even if an output terminal of the output voltage VOUT is ground-faulted, overcurrent protection may be sufficiently activated (refer to FIG. 8B).
However, if an overcurrent protection is temporarily activated due to an event like a change of an AC load, the output transistor 211 is forcefully and unnecessarily turned off for a long period, the output voltage VOUT is stepped down significantly (refer to FIG. 8A).